Currently in the U.S., used or scrap tires are being discarded at a rate in excess of 200 million tires a year, while almost 3 billion tires have already been discarded. Approximately 1.5 waste tires per motor vehicle per year are produced in this country, and only a small fraction of these are used in some way. The most common method of disposal of unwanted tires is land filling, which creates serious environmental problems, and is also a significant loss of resources.
Waste tires are troublesome in sanitary land fills. Whole tires are bulky, and on the average, less than six passenger tires occupy over one cubic yard of land fill space. Whole truck and bus tires occupy even more space. Tires which are compacted in bulk into a land fill tend to spring back to their former shape, and also tend to work up to the surface, or "float" while the fill is settling. As a result, some land fill operators prefer waste tire stockpiling to burying. Stock piling however creates other problems. When tires are to be buried in land fills, the compaction machine or bulldozer operators generally try to spread out the tires that are deposited with the daily supply solid waste, and attempt to bury them under as much other waste as possible. If buried under less than two feet of other waste, the tires will tend to surface during the waste compaction process or the daily cover process. When completing a land fill "cell" or portion of the land fill area, operators try to keep tires out of the last eight to ten feet of compacted waste. If this final segregation of tires is not accomplished, waste tires buried too shallow will often surface or "float" through the final cell cover. It has been reported that waste tires have even surfaced through roads built across completed cells. From an economic standpoint, burying waste tires in land fill requires special operations, and is therefore substantially more costly than land fill of other solid waste materials. Many land fill operators have placed special charges on disposal of tires and in some cases have refused to dispose of tires at all.
As a result, a substantial percentage of waste tires are presently being disposed of through indiscriminate dumping practices. This results in the accumulation of waste tires along roadsides, in ravines, and in wooded areas. In these areas, the tires can serve as a breeding ground for mosquitos, thereby establishing a potential threat to humans through the spread of California Encephalitis. This disease is transmitted through the bite of diseased (virus infected) "tree hole" mosquitos. This particular mosquito, the only variety which carries the virus, lives and breeds in temperate hardwood forests. The treehole mosquito lives in or very near wooded areas and breeds in small pools of water that collect in stumps, hollow trees, or anything that holds water. Waste tires are believed to be a favorite breeding spot because they provide an ideal water container and acts as a natural solar water heater, which can induce earlier breeding in the spring. Spraying via aircraft is ineffective in combating these mosquitos, because they remain sheltered within the hollow spaces of the tire. Discarded tires also provide breeding grounds for rats and other vermin.
Stockpiled tires also present serious fire hazards. Although they are not easily ignited, once burning, tires are difficult to extinguish and produce a thick acrid smoke. It is difficult or impossible for firefighters to travel through or over stacks of tires. In addition, due to their geometry, tires piled up in almost any configuration automatically provide their own air supply. As tires burn, the intense heat given off breaks up the hydrocarbons in the tire material, and convert them back to oil. It is estimated that a completely burned tire yields three to four gallons of oil. The oil itself of course, creates its own environmental and fire hazards. Toxic smoke from tire fires can cause tearing of the eyes, headaches, sore throat, shortness of breath, and vomiting. Tire contents include sulphur, sulphur dichloride, sulphuryl chloride, quinones, and nitro benzines, all of which produce toxic gases or irritants, when burned. In a recent tire fire in Waterbury, Conn., an estimated one million tires collected on a four acre lot burned for about three weeks before the fire department was able to completely extinguish it. The firefighters were forced to wear surgical masks to protect themselves against inhaling the smoke and even then they were not well protected.
In addition to the problems associated with stockpiling of discarded tires as described above, mountains of tires also create an eyesore. Tires are essentially non-biodegradable and therefore present unusual and difficult disposal problems.
In the past, many tires were recapped so that there were many fewer tires being continuously discarded. However, in the past decade the American public has shown an increased preference for purchasing steel belted radial tires over bias belted tires. This trend is generating a tremendous increase in the number of steel belted radial tires which are eventually discarded. The recapping industry has extremely stringent standards for steel belted radial casings. So stringent, in fact, that only a small percentage of this type of passenger car tire completes the recapping cycle. This reduced recapping potential for radials is one source of the increase in waste tires. Most tires presently being discarded are of the steel belted radial design. This aspect is significant because any further processing of waste tires for secondary use must take into consideration the physical properties of the steel belted radial tire and its purposefully designed resistance to cutting, shredding, etc.
Of course, retreading is the preferred way of handling these tires as it not only solves the disposal problem, but it also saves energy and recycles resources. However, presently only approximately 19% of all passenger tires are retreaded, and the trend is downward. Transportation and labor costs combined with lower prices of new tires work against the retreading industry. In addition, truck and especially airplane tires can be retreaded over and over again, but a passenger tire usually can only be retreaded once or twice because of the lower pressure at which it is used. Also about 10% of all tires cannot be retreaded because they represent obsolete styles or sizes for which there is no market. Many smaller retreaders do not attempt to deal with radials because of the large number of molds required to fit radials properly and the inherent problems in radials caused by separation and sidewall distortion. Moreover, retreading of radial tires requires a relatively large number of molds to handle a range of tire sizes, when compared to bias tires. However, there is substantial agreement in the industry that lack of suitable casings is the primary limiting factor in the retreading business. Demand has been healthy in recent years, but perhaps only 60% of all tires removed from road vehicles are inspected for retreadibility. Also, as previously described above, many retreaders are not properly equipped to deal with steel belted radials, which are increasingly penetrating the market.
Rubber reclaiming is another method of disposing of used tires. This industry utilizes one of the oldest technologies available for recycling waste tires and other scrap rubber products. Major reclaiming processes today represent only a slight modification of the processes used at the end of the last century. Although this industry disposes of a significant number of tires annually (approximately ten million in recent years), it faces problems which restrict output and available markets. Specifically these problems are:
(1) A relatively low grade of rubber is produced containing several types of carbon black. PA0 (2) Tire performance standards and increasingly specialized formulations for tire construction have reduced the demand for reclaimed rubber as a component in new tires. PA0 (3) Reclaimed black rubber is not considered aesthetically desirable. PA0 (4) Reclaimed processes are highly labor intensive.
Low operating margins have prevented a significant research and development program which might lead to the production of a superior product with reduced labor and energy requirements. In addition, the continued infringement on the entire rubber products industry by the plastics industry reduces the need for reclaimed rubber. Although reclaiming is significantly less energy intensive than virgin rubber production, the market share of reclaimed rubber has declined steadily dispite higher oil prices. Presently, this method of tire disposal represents at most a few percentage points of all tires disposed.
Similarly, the tire splitting industry does not significantly help in disposing of used tires. The tire splitting industry produces various industrial products die cut from obsolete tires. These products include gaskets, shims, insulators, doormats, and various other items for the automotive industry. It is estimated that presently this industry consumes only 2 million used tires per year. As with rubber reclamation, steel belted radials also present a particular problem for tire splitters and partially account for lack of growth in this industry.
Although present reclaiming processes do not yield rubber of a quality comparable to new rubber, it is possible, by more complex processes, to recover some of the chemical ingredients of tires for use in new synthetic rubber. At least three of such processes are under development. However, none of these processes can operate at a profit, and they have not been applied commercially. Destructive distillation and carbonization are two forms of pyrolysis, a controlled heating process that decomposes materials in the absence of oxygen. Hydrogenization, on the other hand, is a process of chemical synthesis. It involves the addition of hydrogen, the element which is removed from oil to make synthetic rubber, in order to return rubber to its original form. Tires are composed of 83% carbon, 7% hydrogen, and 6% ash, plus small quantities of nitrogen, oxygen, and sulphur. Pyrolysis of tires yields oils, gases, and carbon containing residue. The main difference between destructive distillation and carbonization is the process temperature. At carbonization's higher temperatures, the main product yielded is carbon black, which makes up from 1/4 to 1/3 of the synthetic rubber from which tires are made. The present costs of carbonization are substantially higher than the costs of making carbon black commercially from petroleum. In destructive distillation, as many as fifty gases and liquids are formed, plus a residue consisting mostly of carbon and representing from 35 to 60% of the original weight. The residue could be a high quality fuel, except that it contains 1.5% sulphur. Due to the costs involved, no significant amount of tires are presently reclaimed via pyrolysis or hydrogenization. This alternative use for discarded tires is not yet economically feasible.
Among the more novel uses for used tires is the construction of artificial reefs. Major tire companies have been involved with artificial reef programs on both the east coast and gulf coast. Several thousand reefs have already been built using as many as 3 million tires per reef. In the oceans, whole tires become rapidly encrusted with marine life thus forming a reef attractive to many species of fish. These reefs can be beneficial to commercial fisherman and the recreational fishing industry. The simplest design for reef construction costs from 30 to 40 cents per tire assuming that the tires are delivered on site. The potential use for tires on reefs has been estimated to be in excess of one and a half million tires per year, however, this is less than 0.50% of all tires discarded annually. Also, transportation costs from inland areas pose a significant problem for this use.
Floating tire breakwaters to protect marinas and shore lines subject to erosion have also been developed. Breakwaters have been built and put to use in many areas throughout the country. Presently, it is estimated that breakwaters could use only approximately 250,000 tires per year. As with artificial reefs, transportation costs tend to prevent this type of disposal.
Another rather novel use for used tires is the construction of crash barriers. Used tires have been tested as impact barriers for divider strips, bridge abutments, support posts, guard rails, and overpasses. The barriers are constructed of horizontally stacked tires fastened together with steel cables in tension. Experimental head-on and side angle collisions indicate that the barriers may be effective in reducing injury. However, these barriers may not be suitable for small light-weight vehicles, and additional tests are needed to improve their performance. The barriers have not been approved as a highway hardware item, and it is not known how many used tires these barriers could consume.
Several other uses for used tires are still in the experimental stage. Experiments have been conducted with several micro-organisms which could reduce the size of rubber particles as well as separate oil hydrocarbons from rubber. However, frequently the micro-organisms are supressed by other ingredients in the tire. Ground scrap rubber has also been proposed for utilization as mulch in cultivated fields and in potting soil. The toxic effects, if any, of rubber on plant life in general has not been determined. Crumb rubber, or finely ground rubber, has potential uses in construction as an insulation, pipe coating, or roofing material supplement.
Probably the most promising method of disposing of used tires is converting them into a fuel source. The use of waste tires as a fuel has been approached in two separate technologies, the use of whole tires for fuel, and the use of prepared tires, or tire pieces as fuel. Generally, the systems that have been developed for the incineration of whole tires as a preliminary fuel have encountered substantial problems in the area of incomplete combustion and therefore, their pollutant emissions.
In the area of prepared used tires as a supplementary fuel (mixed with coal), an extensive amount of work has been done. General Motors and the Fisher Body Plant in Pontiac, Mich., completed a development study of the feasibility of burning a mixture of 10% shredded rubber and 90% coal by weight in an industrial size stoker-fired boiler. The equipment used for handling and mixing shredded rubber with the coal performed satisfactorily. However, the rubber tended to jam in certain machinery. There were no operation or deterioration problems with the boilers. Stack emission tests revealed compliance with both the Michigan State particulate and SO.sub.2 codes. The rubber-coal fuel mixture represented a yearly savings of approximately 6% of the powerhouse fuel costs, not including capitalization costs on the rubber handling equipment. In the General Motor's tests, shredded rubber was mixed directly with the coal through the coalcar unloading grates. While air pollution emission testing indicated an increase in particulate emissions when the rubber and coal mixture was burned, the emission levels were still within code. The low cost of coal at that time (1970) and the relatively high cost of shredding tires, plus the material problems associated with shredded rubber, caused the project to be discontinued.
During the approximate six months that the coal-rubber mixture was burned a total of five particulate emission tests were carried out on various boilers burning the 10% rubber and 90% coal mixture. The average particulate increase over coal firing was 62.4%. Of this increase, 15% was from zinc oxide, with the remaining 47.4% in particulate emission assumed to be composed of tire belting material, other structural material in the tires, and grinding fines. Sulphur dioxide emissions actually decreased as result of burning the mixed fuel. The rubber, which has a low sulphur content of approximately 1% and high BTU content per pound, decreases the amount of SO.sub.2 generated per BTU of heat release. Due to the high temperatures and the excess air used in the industrial stoker fired boilers, there were no odor problems encountered in burning the shredded scrap rubber.
In May 1985 emission tests were conducted at a Ford Motor Company boiler in Brookpark, Ohio, which was burning a mixture of approximately 15% recycled rubber and 85% coal. These tests revealed that the levels of particulate emissions were significantly below the Environmental Protection Agency permissible level.
The use of shredded scrap tires in road building appears to be another promising way of disposing of used tires. There is a considerable amount of historic precedence for using pieces of scrap tires in road building as an aggregate for the road surface itself and also as an asphalt additive or binder. Evidence shows that the rubber improves the road surface in terms of wear and also seems to make a good patching compound. Although most experimenting with rubberized roads has been completed in Europe, some 10,000 miles of rubberized roads have already been built in the U.S. Specialized surfaces such as bridge pavement, tennis courts, playgrounds, and running tracks have successfully included rubber compounds. However, a majority of these surfaces have been made with newly manufactured synthetic rubber compounds rather than old rubber from these tires. Compounds made primarily from re-claimed tire rubber have been used in road surfacing in at least 52 projects in 9 states. A report by the Engineers Office for the City of Phoenix, indicated that after 11 years, the asphalt rubber system used in road paving projects there was performing well. The paving material used in Phoenix was composed of hot asphalt cement mixed with 25% ground tire rubber and diluted with kerosene for easy application, or by an alternative process including hot asphalt cement mixed with 18 to 22% ground rubber and diluted with an oil extender. The report stated that the asphalt rubber concept has proven the value of a truly flexible and elastic member as a crack reflection preventative. The engineers report concluded that "the use of asphalt rubber also will be continued because of its sound engineering properties, economic advantages and success as a surface for our streets, highways, and airports."
Adding shredded used tires to paving material also saves energy. Although the cost of asphalt rubber is initially higher than the customary bituminous road paving materials, its life expectancy is approximately 5 times greater than the non-rubber containing material, thereby offsetting the higher cost. Ground rubber used in paving materials must be free of any foreign matter (metals and fibers) and have a mesh size of minus 16 to plus 25. To pave one lane mile with asphalt 2" thick requires over 300 gallons of petroleum. Assuming a rubber-asphalt system lasts three times longer than a standard asphalt highway, then 300 gallons of oil are saved over that period of time per lane mile. In 1978, there were over 600,000 lane miles paved with asphalt in the United States. Had the rubber-asphalt system been employed, almost 200 million gallons of petroleum would have been conserved, based on the figures above. A recently enacted federal law makes it mandatory to use recycled rubber granules, subject to availability, on all new federal road construction or repair.
As described above, tires may be used on a large scale as a source of fuel. However, they must be first adequately prepared to insure a successful burning operation. Test results indicate that whole tires do not make a good fuel source but that tire pieces can be used as a valuable supplemental fuel. It has been shown that the size of a chip or portion of tire to be used as fuel in a stoker-fired boiler designed for coal should be about 2". Tests at the General Motors plant concluded that tire fines (0 to 1/8" pieces) are not desirable because of the amount of suspension burn and resulting emissions. Therefore, a used tire must be processed down to about 11/2 to 2" chips for use as a supplemental fuel in a stoker-fired boiler. Similarly, rubber for use in road paving materials must be in the form of small granules.
Three used tire size reduction processing systems, i.e. mechanical, cryogenic-mechanical, and cryogenic have been used. Mechanical size reduction involves slicing or beating the tire into small strips or chunks. This type of size reduction is the least expensive per tire and can produce a compatible feed material for stoker-fired spreader boilers. The cryogenic and cryogenic-mechanical size reduction systems are quite similar to each other. The cryogenic-mechanical system operates on a chunk of strip feed, whereas the straight cryogenic system accepts whole tires. Both of these systems operate on the principle of lowering the temperature of the rubber below its brittle temperature by spraying or dipping the tire in some cryogenic substance and then crushing the frozen material. Dry ice, dry ice with methanol, and liquid nitrogen, have all been used as cryogenic materials. Liquid nitrogen, with a boiling point of -196.degree. C., is most commonly used. The use of liquid nitrogen in a cryogenic-mechanical system is more efficient than the other cryogenic materials, because of its larger surface-area-to-volume ratio. In addition, after freezing, a smaller hammermill can be used, reducing capital and operating costs.
With all three of the tire reduction systems, the chip size and impurity content of the product must be controlled, if it is to be useable as a fuel or paving material source. If the rubber for fuel is not prepared to a consistant size within the specification range, and oversized chips enter the fuel system of a boiler, mechanical problems may arise. The large chips of rubber do not have the consistancy of coal, and therefore, do not "crush out" between mechanisms. This may result in flow blockage or mechanical breakdown in the fuel feed mechanical system. Alternatively, if the fuel has a high percentage of fines (small particles of rubber), problems may result in excessive super heater temperatures or excessive loading of air polution equipment as a result of suspension burning.
In addition to rubber, tires include steel or fiber products, which normally must be removed from rubber which is to be used as a fuel or paving material source. A typical steel belted passenger tire contains approximately 5 lbs. of steel. Fabric tires may include rayon, nylon, or polyester, in addition to a steel bead. The steel and fibers must normally be removed in order to provide a fuel which can be efficiently fed into a stoker and burned. In some boilers however, rubber chunks containing fibers and steel can be used, with the fibers being combusted with the rubber and the steel being removed afterwards from the grates.
Although the tire recycling concept is not new the previously used recycling methods worked well only on fabric tires, and had only limited ability to handle steel belted tires and truck tires. In addition, the conventional hammermill methods involve relatively high costs, are not energy efficient, and cannot handle large quantities of tires effectively. Moreover, existing methods have required manual separation of fibers and steel.
Accordingly, it is an object of the present invention to provide an improved process for converting used tires into a valuable rubber resource having a variety of uses.
It is also an object of the invention to provide such a process which also automatically extracts steel and fiber material from used tires for recycling.